Modern semiconductor processing is typically performed using at least one photolithographic step involving an apparatus that projects a pattern, defined by a mask or reticle, onto the surface of a photosensitive substrate (e.g., "wafer"). Semiconductor devices manufactured using such an apparatus include, for example, integrated circuits, microprocessors, memories, liquid-crystal display devices, image-pickup devices (CCDs, etc.), and thin-film magnetic heads. Such apparatus are usually termed "aligners" or "steppers". Virtually all aligners and steppers comprise an apparatus operable to properly align the photosensitive substrate with the mask before an exposure is made.
More specifically, steppers and other projection-exposure apparatus typically comprise an alignment apparatus operable to ensure high-precision overlay registration of the mask with the substrate. To facilitate such alignment, alignment marks are provided at each exposure region ("die") on the wafer.
Contemporary alignment apparatus typically exploit one of the following technologies: (1) LSA (Laser-Step Alignment), wherein laser light is irradiated on an alignment mark configured as an array of dots on the wafer, and the position of the mark is detected using light that has been diffracted or scattered by the mark; (2) FIA (Field Image Alignment), involving processing of image data picked up from an alignment mark on the wafer illuminated by light produced by a halogen lamp source and having a broad spectral bandwidth; and (3) LIA (Laser Interferometric Alignment), wherein an alignment mark, in the form of a diffraction grating, on the wafer is irradiated from two directions by laser light of the same or slightly different frequencies, the resulting two beams of diffracted light are caused to interfere with one another and the position of the alignment mark is measured from the phases of such interference.
Conventional alignment methods typically fall into one of the following categories: (1) TTL (Throughthe-Lens), wherein the position of an alignment mark on the wafer is measured by passing light from the alignment mark through a projection optical system; (2) Off-Axis, wherein the position of an alignment mark on the wafer is measured directly without passing light from the alignment mark through a projection optical system; and (3) TTR (Through-the-Reticle), wherein both the wafer and the reticle (mask) are simultaneously viewed via a projection optical system and the relative positional relationship between the mask and wafer is detected.
When performing alignment of a reticle and wafer through the use of one of the foregoing, the magnitude of a baseline, which is the distance between the center of measurement of the alignment sensor and the center of the projected image of the reticle pattern (i.e., the center of exposure), is determined in advance. Then, by using the alignment sensor to detect the offset of the alignment mark from the measurement center, and by moving the wafer a distance equal to the offset after correction for the baseline, the center of the exposure region in question is accurately aligned to the center of exposure.
However, over the course of continued use of the aligner, a gradual change in the magnitude of the baseline can arise. Such "baseline drift" can cause a decline in alignment precision (i.e., the precision of overlay registration). As a preventative measure, periodic baseline checks are usually performed, usually to ensure accurate measurements of the distance between the alignment-sensor measurement center and the exposure center.
However, despite periodic baseline checks, declines in alignment precision can arise during the intervals between baseline checks if the baseline drifts over shorter periods. One factor responsible for such short-period baseline drifts or other baseline fluctuations is displacement of the alignment-sensor measurement center position concomitant with mechanical vibration or thermal deformation resulting from irradiation by the illuminant light for exposure, atmospheric changes, or other environmental changes. Thus, even if the alignment sensor and the wafer are actually stationary with respect to each other, such drift can cause a change in positional offset between the measurement center and the alignment mark, resulting in alignment error.
The resistance to change in the positional offset between the measurement center of an alignment sensor and the alignment mark which it measures is referred to herein as the "drift stability" of the alignment sensor.
Because off-axis-type alignment sensors do not use a projection optical system in detecting the alignment mark on the wafer, it is important (more important than with TTL-type and other alignment sensors that do use a projection optical system) to improve the drift stability of off-axis-type alignment sensors as much as possible.
In recent years, to accommodate a trend toward finer linewidths (feature sizes) for semiconductor devices and the like, increasingly shorter wavelengths of illumination light have been required to attain satisfactory high resolution. I.e., ultraviolet illumination light is increasingly being used, including far ultraviolet light generated by, e.g., KrF excimer lasers or ArF excimer lasers. Projection apparatus using, e.g., excimer lasers as the exposure illuminant light increasingly employ off-axis-type alignment sensors because of their liberal degrees of freedom in design and high potential performance. (TTL-type alignment sensors experience myriad technical difficulties when used with such projection apparatus.) However, unless the drift stability of an off-axis-type alignment sensor is high, undesirable declines in alignment precision will arise compared to the precision of alignment realized using TTL or other alignment methods.